U.S. patent application number 11/579456 was filed with the patent office on 2008-11-06 for catalysts.
Invention is credited to John Leonello Casci, Carl Leonard Huitson, Cornelis Martinus Lok.
Application Number | 20080275145 11/579456 |
Document ID | / |
Family ID | 32482629 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080275145 |
Kind Code |
A1 |
Casci; John Leonello ; et
al. |
November 6, 2008 |
Catalysts
Abstract
A catalyst including cobalt, zinc oxide and aluminium is
described, having a total cobalt content of 15-75% by weight (on
reduced catalyst), an aluminium content .gtoreq.10% by weight
(based on ZnO) and which when reduced at 425.degree. C., has a
cobalt surface area as measured by hydrogen chemisorption at
150.degree. C. of at least 20 m.sup.2/g cobalt. A method for
preparing the catalyst is also described including combining a
solution of cobalt, zinc and aluminium with an alkaline solution to
effect co-precipitation of a cobalt-zinc-aluminium composition from
the combined solutions, separating of the co-precipitated
composition form the combined solutions, heating the composition to
form an oxide composition, and optionally reducing at least a
portion of the cobalt to cobalt metal. The catalysts may be used
for hydrogenation reactions and for the Fischer-Tropsch synthesis
of hydrocarbons.
Inventors: |
Casci; John Leonello;
(Cleveland, GB) ; Huitson; Carl Leonard;
(Cleveland, GB) ; Lok; Cornelis Martinus;
(Cleveland, GB) |
Correspondence
Address: |
RATNERPRESTIA
P O BOX 980
VALLEY FORGE
PA
19482-0980
US
|
Family ID: |
32482629 |
Appl. No.: |
11/579456 |
Filed: |
April 15, 2005 |
PCT Filed: |
April 15, 2005 |
PCT NO: |
PCT/GB2005/01468 |
371 Date: |
November 3, 2006 |
Current U.S.
Class: |
518/717 ;
502/303; 502/304; 502/327; 502/329; 518/715; 564/490; 568/885;
585/276 |
Current CPC
Class: |
B01J 23/34 20130101;
B01J 23/8892 20130101; B01J 35/1042 20130101; B01J 23/80 20130101;
B01J 35/0053 20130101; C10G 45/00 20130101; C10G 2/332 20130101;
B01J 37/18 20130101; B01J 37/03 20130101; B01J 23/10 20130101; B01J
35/10 20130101 |
Class at
Publication: |
518/717 ;
502/329; 502/303; 502/304; 502/327; 568/885; 564/490; 585/276;
518/715 |
International
Class: |
B01J 21/00 20060101
B01J021/00; C07C 29/14 20060101 C07C029/14; C07C 209/48 20060101
C07C209/48; C07C 5/02 20060101 C07C005/02; C07C 27/06 20060101
C07C027/06; B01J 21/04 20060101 B01J021/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 4, 2004 |
GB |
0409901.6 |
Claims
1. A catalyst consisting of cobalt, zinc oxide and aluminium having
a total cobalt content of 15-75% by weight (on reduced catalyst),
an aluminium content .gtoreq.10% by weight (based on ZnO) and which
when reduced at 425.degree. C., has a cobalt surface area as
measured by hydrogen chemisorption at 150.degree. C. of at least 20
m.sup.2/g of cobalt.
2. A catalyst according to claim 1 wherein the cobalt content is
.gtoreq.20% by weight (on reduced catalyst).
3. (canceled)
4. A catalyst according to claim 1 wherein the cobalt surface area
following reduction at 425.degree. C. is >40 m.sup.2/g of
cobalt.
5. A catalyst according to claim 1 wherein the pore volume of the
reduced catalyst is >0.5 ml/g catalyst.
6. A method for preparing a catalyst according to claim 1
comprising the steps of: (i) combining a solution of cobalt, zinc
and aluminium with an alkaline solution to effect co-precipitation
of a cobalt-zinc-aluminium composition from the combined solutions,
(ii) separating the co-precipitated composition from the combined
solutions, and (iii) heating the composition to form an oxidic
composition.
7. A method according to claim 6 wherein the cobalt, zinc and
aluminium solution and alkaline solution are added simultaneously
to a stirred vessel.
8. A method according to claim 6 wherein particulate alumina or
hydrous alumina is added to the combined solution during the
co-precipitation step.
9. A process for hydrogenating a compound comprising treating the
compound with a hydrogen-containing gas under pressure in an
autoclave at ambient or elevated temperature in the presence of the
catalyst according to claim 1.
10. A process for the Fischer-Tropsch synthesis of hydrocarbons
comprising converting a mixture of carbon monoxide and hydrogen
hydrocarbons in the presence of a catalyst according to claim
1.
11. A method according to claim 6 further comprising the step of:
(iv) reducing at least a portion of the cobalt to cobalt metal.
12. A catalyst consisting of cobalt, zinc oxide and aluminium and
one or more promoters selected from the group consisting of
magnesia, ceria lanthana, titania, zirconia, hafnia, ruthenium,
platinum, palladium or rhenium or a combination thereof, having a
total cobalt content of 15-75% by weight (on reduced catalyst), an
aluminium content .gtoreq.10% by weight (based on ZnO) and which
when reduced at 425.degree. C., has a cobalt surface area as
measured by hydrogen chemisorption at 150.degree. C. of at least 20
m.sup.2/g of cobalt.
13. A catalyst according to claim 12 wherein the cobalt content is
.gtoreq.20% by weight (on reduced catalyst).
14. A catalyst according to claim 12 wherein the cobalt surface
area following reduction at 425.degree. C. is >40 m.sup.2/g of
cobalt.
15. A catalyst according to claim 12 wherein the pore volume of the
reduced catalyst is >0.5 ml/g catalyst.
16. A method for preparing a catalyst according to claim 12
comprising the steps of: (i) combining a solution of cobalt, zinc
and aluminium with an alkaline solution to effect co-precipitation
of a cobalt-zinc-aluminium composition from the combined solutions,
(ii) separating the co-precipitated composition from the combined
solutions, and (iii) heating the composition to form an oxidic
composition.
17. A method according to claim 16 further comprising the step of:
(iv) reducing at least a portion of the cobalt to cobalt metal.
18. A method according to claim 16 wherein the cobalt, zinc and
aluminium solution and alkaline solution are added simultaneously
to a stirred vessel.
19. A method according to claim 16 wherein particulate alumina or
hydrous alumina is added to the combined solution during the
co-precipitation step.
20. A process for hydrogenating a compound comprising treating the
compound with a hydrogen-containing gas under pressure in an
autoclave at ambient or elevated temperature in the presence of the
catalyst according to claim 12.
21. A process for the Fischer-Tropsch synthesis of hydrocarbons
comprising converting a mixture of carbon monoxide and hydrogen
hydrocarbons in the presence of a catalyst according to claim 12.
Description
[0001] This invention relates to cobalt catalysts and in particular
to cobalt catalysts comprising cobalt, zinc and aluminium.
[0002] Supported cobalt catalysts wherein the cobalt is in its
elemental or reduced state are well known and find use in many
reactions involving hydrogen such as hydrogenation reactions, e.g.
nitrile hydrogenation reactions and the Fischer-Tropsch synthesis
of hydrocarbons. The activity of the catalysts is believed to be
directly proportional to the cobalt surface area of the reduced
catalysts, but in order to achieve high cobalt surface areas, the
cobalt should be well dispersed on the support. As the cobalt
content of a catalyst increases above 15%, particularly above 20%
by weight (on reduced catalyst) the cobalt becomes more difficult
to disperse resulting in lower cobalt surface areas per gram
cobalt. Cobalt is a relatively expensive metal and therefore there
is a desire to improve the cobalt dispersion. (expressed as cobalt
surface area per gram cobalt) for supported cobalt catalysts.
[0003] Cobalt catalysts comprising cobalt on zinc oxide/aluminium
compositions are known. EP-B1-0671976 describes cobalt/zinc oxide
catalysts wherein the zinc oxide contains a group IIIa metal such
as aluminium at 0.5-7.5% preferably 0.8-2% by weight (as metal on
weight of zinc oxide). The low levels of aluminium, present in the
composition in the form of a spinel (i.e. ZnAl.sub.2O.sub.4), were
apparently included to increase the compression strength or reduce
shrinkage and sintering. Whereas the cobalt content of the Co/Zn/Al
catalysts is given as 3-40% wt (Co metal on total weight of
composition), only 10% wt was exemplified and no indication of the
cobalt surface area was given. Furthermore, the catalyst
preparation method exemplified required firstly co-precipitating
the zinc oxide composition, filtering, washing, drying and
calcining it, then slurrying the calcined composition with cobalt
nitrate solution, drying the slurry, grinding and re-calcining. The
multiple heating steps in this method make it unattractive for
large scale catalyst manufacture. Consequently a method whereby a
Co/Zn/Al catalyst is formed in fewer steps, in particular without
the need for two-calcination steps is desirable.
[0004] EP-A-1358934 describes particulate cobalt-zinc
co-precipitated catalysts having a volume average particle size of
less than 150 .mu.m and their use for the Fischer-Tropsch synthesis
of hydrocarbons. Whereas it is stated that a group IIIa element
such as aluminium, may be present in a concentration of 0.1-10% wt
(on catalyst) to effect structural stability, no specific
disclosure of aluminium-containing catalysts is made, nor is any
data provided on the cobalt surface areas of the resulting
catalysts.
[0005] We have found that the methods described heretofore do not
provide the desired high cobalt dispersions and hence high cobalt
surface areas per gram cobalt. We have found that the incorporation
of aluminium at levels .gtoreq.10% wt (based on ZnO) in cobalt-zinc
oxide catalysts having a cobalt content in the range 15-75% wt has
a beneficial effect on the cobalt dispersion and resulting cobalt
surface area.
[0006] Accordingly, the present invention provides a catalyst
comprising cobalt, zinc oxide and aluminium having a total cobalt
content of 15-75% by weight (on reduced catalyst), an aluminium
content .gtoreq.10% by weight (based on ZnO) and which when reduced
at 425.degree. C., has a cobalt surface area as measured by
hydrogen chemisorption at 150.degree. C. of at least 20 m.sup.2/g
of cobalt.
[0007] The invention further provides method for preparing the
above catalyst comprising cobalt, zinc oxide and aluminium wherein
the cobalt content of the catalyst is 15-75% by weight (on reduced
catalyst), by [0008] (i) combining a solution of cobalt, zinc and
aluminium with an alkaline solution to effect co-precipitation of a
cobalt-zinc-aluminium composition from the combined solutions,
[0009] (ii) separating of the co-precipitated composition from the
combined solutions, [0010] (iii) heating the composition to form an
oxidic composition, and optionally [0011] (iv) reducing at least a
portion of the cobalt to cobalt metal [0012] wherein the acidic
solutions are combined in amounts such that aluminium content of
the oxidic composition is .gtoreq.10% by weight (based on ZnO), and
which when reduced at 425.degree. C., has a cobalt surface area as
measured by hydrogen chemisorption at 150.degree. C. of at least 20
m.sup.2/g of cobalt.
[0013] The invention also provides the use of the catalysts for
hydrogenation reactions and for the Fischer-Tropsch synthesis of
hydrocarbons.
[0014] The catalyst comprises cobalt species intimately mixed with
aluminium-containing zinc oxide in which the aluminium may be in
the form of alumina and/or ZnAl.sub.2O.sub.4. The term "cobalt
species" is used broadly to include both elemental cobalt and
cobalt in combined form, e.g. as compounds such as cobalt oxides
and cobalt hydroxycarbonates. The catalyst in its reduced form is
useful for catalysing hydrogenation reactions and the
Fischer-Tropsch synthesis of hydrocarbons. The catalyst may,
however, be provided as an unreduced precursor wherein the cobalt
is present as one or more compounds, such as oxides or
hydroxycarbonates, reducible to elemental cobalt. The reduction of
the cobalt may then be performed by the user in-situ. The cobalt
surface area figures used herein, unless otherwise stated, apply to
the material after reduction, but the invention is not limited to
the provision of reduced catalyst.
[0015] Zinc oxide is a useful alternative catalyst support. For
example, it may offer improvements with regard to the physical
properties or reducibility of the resulting cobalt species compared
to other catalyst supports. Furthermore zinc oxide is able to act
as a catalyst poison `sink` e.g. for any sulphur compounds which
may be present in the process gas streams. Protecting the catalyst
from sulphur poisoning can extend the useful life of the
catalyst.
[0016] The cobalt content of the catalysts is in the range 15-75%,
preferably 220% by weight (on reduced catalyst). For hydrogenation
or Fischer-Tropsch catalysts, it is desirable to provide cobalt
levels .gtoreq.15%, particularly .gtoreq.20% by weight of cobalt to
reduce the volume of catalyst and hence the size of process
equipment. Furthermore, where the catalyst support is able to form
cobalt compounds that are difficult to reduce, e.g. cobalt spinels,
by providing higher levels of cobalt, the negative effect of spinel
formation is reduced because more of the cobalt is available for
subsequent reduction.
[0017] The catalysts comprise aluminium in the form of alumina
and/or ZnAl.sub.2O.sub.4. The presence of any ZnAl.sub.2O.sub.4 may
be determined by X-ray diffraction. Preferably the aluminium is
predominantly present as alumina (Al.sub.2O.sub.3). The aluminium
content of the catalyst is .gtoreq.10%, preferably >20%,
especially >25%, by weight (based on ZnO). Preferably the
aluminium content of the catalyst is .ltoreq.66%, more preferably
.ltoreq.33% by weight (based on ZnO).
[0018] The catalyst may also contain other components that improve
the catalyst physical properties or its susceptibility to
reduction. For example, the catalyst may contain one or more
promoters such as magnesia, ceria lanthana, titania, zirconia,
hafnia, ruthenium, platinum, palladium or rhenium or a combination
thereof. Preferably the catalyst, when formulated for the
Fischer-Tropsch synthesis of hydrocarbons, comprises one or more
promoters selected from the above list. In one preferred embodiment
the catalyst comprises cobalt, zinc, alumina and magnesia. The
amount of promoter, if present, is preferably less than 10 mole %
based on cobalt.
[0019] Catalysts of the present invention may be prepared by
co-precipitating a cobalt-zinc-aluminium composition. This is may
be accomplished by addition of an aqueous alkaline solution to a
stirred, combined solution of cobalt, zinc and aluminium or by
adding the combined cobalt/zinc/aluminium solution to the stirred
alkaline solution, but is preferably accomplished by combining the
Co/Zn/Al and alkaline solutions simultaneously to a stirred
precipitation reactor, which may contain an aqueous medium, e.g.
water. The Co/Zn/Al solution may be prepared by dissolving cobalt,
zinc and aluminium separately or together in acid solution, e.g.
nitric or acetic acid or by dissolving suitable soluble cobalt,
zinc and aluminium compounds in water or dilute acid. Preferred
soluble cobalt and zinc compounds are the acetates and nitrates,
particularly the nitrates. Suitable soluble aluminium compounds are
sodium aluminate and aluminium nitrate. Preferably the pH of the
solutions is adjusted if necessary to prevent premature
precipitation.
[0020] The alkaline solution may be a solution of an organic base
and/or an inorganic base. Organic bases include tetraalkyl ammonium
hydroxides, amines, pyridine or alkanolamines. Inorganic bases
include aqueous ammonia, ammonium carbonate, ammonium bicarbonate
and/or a group I or group II metal hydroxide, bicarbonate or
carbonate such as sodium hydroxide, sodium carbonate, sodium
bicarbonate, potassium hydroxide or potassium carbonate. Inorganic
bases are preferred. The concentration of the various components is
typically in the range 0.1 to 5 moles/litre and may be selected
appropriately to suit the equipment and scale of operation using
knowledge known to those skilled in the art.
[0021] Where promoters are present in the catalyst, these may be
included by addition of suitable precursor compounds such as metal
salts, e.g. metal nitrates or metal acetates, or suitable
metal-organic compounds, such as metal alkoxides or metal
acetylacetonates, to the combined solution before or during the
co-precipitation.
[0022] The shift in pH on combining the Co/Zn/Al and alkaline
solutions causes a cobalt-zinc-aluminium composition to precipitate
from the combined solution. The pH of the Co/Zn/Al solution is
preferably in the range 1-5 and the pH of the alkaline solution is
preferably in the range 7-14. Where co-precipitation is effected by
simultaneous addition of the Co/Zn/Al and alkaline solutions to an
aqueous medium, the pH of said medium is preferably in the range
4-9. The pH at which precipitation occurs will vary with the exact
composition, but is preferably >5, particularly 6.5-7.5. It is
preferred that the pH during precipitation is maintained at a
constant value (.+-.0.2 pH units) by controlling the addition of
the Co/Zn/Al and alkaline solutions to the precipitation reactor.
Furthermore and the speed of stirring and the temperature at which
the precipitation is performed may be increased or reduced during
precipitation influence the properties of the resulting catalyst.
Preferably the co-precipitation is performed at a temperature
between 15 and 100.degree. C., preferably >50.degree. C.
[0023] In a preferred embodiment, alumina and/or preferably a
hydrous alumina, especially alumina trihydrate, is added to the
combined solution before, but preferably during, the
co-precipitation. Providing aluminium in this way allows variation
in the catalyst properties.
[0024] The pore volume of the reduced catalyst of the present
invention is preferably >0.5 ml/g catalyst.
[0025] The resulting co-precipitated composition is separated from
the combined solutions, typically by filtration or centrifugation
to yield a damp product, which is preferably washed with water to
remove traces of soluble salts. The separated composition is then
subjected to heating to form an oxidic composition. The heating
step is generally accomplished by drying and calcining the
composition. This may be performed in one or two stages. Drying is
typically performed at 60-130.degree. C. Calcination may be
performed at 250-600.degree. C. or higher for up to 24 hours, but
is preferably performed at 250-450.degree. C. over 1-10 hours. The
resulting oxidic composition comprises an intimate mixture of
cobalt, zinc and aluminium oxides, possibly with residual carbonate
species, in which the aluminium is present in an amount .gtoreq.10%
by weight (based on ZnO). To render the catalyst catalytically
active for Fischer-Tropsch or hydrogenation reactions, at least a
portion of the cobalt is preferably reduced to cobalt metal.
Reduction may be performed using hydrogen-containing gasses at
elevated temperature.
[0026] Before the reduction step, the oxidic composition may, if
desired, be formed into shaped units suitable for the process for
which the catalyst is intended, using methods known to those
skilled in the art. The shaped units may be spheres, pellets,
cylinders, rings, or multi-holed pellets, which may be multi-lobed
or fluted, e.g. of cloverleaf cross-section.
[0027] The oxidic composition may be reduced to provide cobalt in
the elemental state. Reduction may be performed by passing a
hydrogen-containing gas such as hydrogen, synthesis gas or a
mixture of hydrogen with nitrogen or other inert gas over the
oxidic composition at elevated temperature, for example by passing
the hydrogen-containing gas over the composition at temperatures in
the range 150-500.degree. C. for between 1 and 16 hours, preferably
300-500.degree. C. Catalysts in the reduced state can be difficult
to handle as they can react spontaneously with oxygen in air, which
can lead to undesirable self-heating and loss of activity.
Consequently reduced catalysts suitable for hydrogenation reactions
are preferably passivated following reduction with an
oxygen-containing gas, often air or oxygen in carbon dioxide and/or
nitrogen. Passivation provides a thin protective layer sufficient
to prevent undesirable reaction with air, but which is readily
removed once the catalyst has been installed in a hydrogenation
process by treatment with a hydrogen-containing gas. For catalysts
suitable for Fischer-Tropsch processes, passivation is not
preferred and the reduced catalyst is preferably protected by
encapsulation of the reduced catalyst particles with a suitable
barrier coating. In the case of a Fischer-Tropsch catalyst, this
may suitably be a FT-hydrocarbon wax. Alternatively, the catalyst
can be provided in the oxidic unreduced state and reduced in-situ
with a hydrogen-containing gas. Whichever route is chosen, the
catalysts of the present invention provide high cobalt surface
areas per gram cobalt. The catalysts, when reduced at 425.degree.
C., have a cobalt surface area of at least 20 m.sup.2/g of cobalt
as measured by the H.sub.2 chemisorption technique described
herein. Preferably the cobalt surface area is greater than 30
m.sup.2/g, more preferably at least 40 m.sup.2/g. Preferably, in
order to achieve a suitable catalyst volume in hydrogenation and or
Fischer-Tropsch processes, the catalysts will have a cobalt surface
area/g catalyst >10 m.sup.2/g catalyst, more preferably >15
m.sup.2/g catalyst.
[0028] The cobalt surface area is determined by H.sub.2
chemisorption. The preferred method is as follows; approximately
0.2 to 0.5 g of sample material is firstly degassed and dried by
heating to 140.degree. C. at 1.degree. C./min in flowing helium and
holding it at 140.degree. C. for 60 mins. The degassed and dried
sample is then reduced by heating it from 140.degree. C. to
425.degree. C. at a rate of 3.degree. C./min under a 50 ml/min flow
of hydrogen and then holding it under the same hydrogen flow, at
425.degree. C. for 6 hours. Following reduction and under vacuum,
the sample is heated up to 450.degree. C. at 10.degree. C./min and
held under these conditions for 2 hours. The sample is then cooled
to 150.degree. C. and held for a further 30 minutes under vacuum.
The chemisorption, analysis is carried out at 150.degree. C. using
pure hydrogen gas. An automatic analysis program is used to measure
a full isotherm over the range 100 mmHg up to 760 mmHg pressure of
hydrogen. The analysis is carried out twice; the first measures the
"total" hydrogen uptake (i.e. includes chemisorbed hydrogen and
physisorbed hydrogen) and immediately following the first analysis
the sample is put under vacuum (<5 mm Hg) for 30 mins. The
analysis is then repeated to measure the physisorbed uptake. A
linear regression may then be applied to the "total" uptake data
with extrapolation back to zero pressure to calculate the volume of
gas chemisorbed (V).
[0029] Cobalt surface areas were calculated in all cases using the
following equation;
Co surface
area=(6.023.times.10.sup.23.times.V.times.SF.times.A)/22414 [0030]
where [0031] V=uptake of H.sub.2 in ml/g [0032] SF=Stoichiometry
factor (assumed 2 for H.sub.2 chemisorption on Co) [0033] A=area
occupied by one atom of cobalt (assumed 0.0662 nm.sup.2)
[0034] This equation is described in the Operators Manual for the
Micromeretics ASAP 2010 Chemi System V 2.01, Appendix C, Part No.
201-42808-01, October 1996.
[0035] The catalysts of the present invention may be used for
hydrogenation reactions and for the Fischer-Tropsch synthesis of
hydrocarbons.
[0036] Typical hydrogenation reactions include the hydrogenation of
aldehydes and nitriles to alcohols and amines respectively, and the
hydrogenation of cyclic aromatic compounds or unsaturated
hydrocarbons. Such hydrogenation reactions are typically performed
in a continuous or batch-wise manner by treating the compound to be
hydrogenated with a hydrogen-containing gas under pressure in an
autoclave at ambient or elevated temperature in the presence of the
cobalt-catalyst.
[0037] The Fischer-Tropsch synthesis of hydrocarbons is well
established. The Fischer-Tropsch synthesis converts a mixture of
carbon monoxide and hydrogen to hydrocarbons. The mixture of carbon
monoxide and hydrogen is typically a synthesis gas having a
hydrogen:carbon monoxide ratio in the range 1.7-2.5:1. The reaction
may be performed in a continuous or batch process using one or more
stirred slurry-phase reactors, bubble-column reactors, loop
reactors or fluidised bed reactors. The process may be operated at
pressures in the range 0.1-10 Mpa and temperatures in the range
150-350.degree. C. The gas-hourly-space velocity (GHSV) for
continuous operation is in the range 100-25000 hr.sup.-1. The
catalysts of the present invention are of particular utility
because of their high cobalt surface areas/g cobalt.
[0038] The invention will now be further described by reference to
the following examples.
EXAMPLE 1
Preparation of Catalysts
[0039] A Co/Zn/Al solution was prepared by dissolving sodium
aluminate (52.5 g) in 1 litre of cold demineralised water. To this
was added 250 ml of nitric acid with stirring. Cobalt nitrate
nonahydrate (1281 g) and zinc nitrate hexahydrate (729.4 g) were
then dissolved in hot demineralised water and added to the sodium
aluminate solution with stirring. This solution was then made up to
a final volume of 3.7 litres with demineralised water. A 1.5 molar
sodium carbonate solution in water (4.2 litres) was separately
prepared.
[0040] (a) 2.2 litres of the Co/Zn/Al solution and the sodium
carbonate solution were heated to 80-85.degree. C. and pumped
simultaneously to a stirred precipitation reactor. The rate of
addition was controlled so that the pH in the combined solution was
pH 6.8. Slurried alumina trihydrate (60 g in 300 ml water) was
added during the co-precipitation to the stirred reaction mixture.
Once the additions were completed, the cobalt-zinc-aluminium
composition was recovered by filtration and washed with hot
demineralised water to remove traces of sodium. The filtrate was
colourless indicating complete precipitation of the cobalt. The
filter cake was then dried at 120.degree. C. for 16 h, followed by
calcination at 300.degree. C. for 6 h.
[0041] (b) The above method was repeated using the remaining 1.5
litres of the Co/Zn/Al solution but increasing the alumina
trihydrate addition to 128.1 g in 600 ml water.
[0042] The compositions of the un-reduced catalysts were as
follows;
TABLE-US-00001 Wt % Example 1a Example 1b Co 35.4 37.5 Zn 21.7 22.7
Al 3.2 6.7 % Al on ZnO 11.8 23.6
[0043] The materials were subjected to reduction at 425.degree. C.
and cobalt surface area analysis using the method described above.
In addition, using nitrogen physisorption, the BET surface area,
pore volume and pore diameter were measured on the oxidic
composition and on the reduced catalyst. The analytical results are
as follows;
TABLE-US-00002 Co Surface Ex- WLOR Co in reduced Co Surface Area
Area ample (% w/w) form (Wt %) (m.sup.2/g catalyst) (m.sup.2/g Co)
1a 25 48.3 18.1 37.5 1b 22 42.4 22.7 53.6
[0044] WLOR=weight lost on reduction. The results demonstrate that
the catalyst of the present invention has cobalt surface areas/g
cobalt >20 m2/g.
TABLE-US-00003 BET Surface area Pore Volume Average Pore Example
(m.sup.2/g) (ml/g) Diameter (.ANG.) 1a 173.8 0.67 160 Oxidic
Composition 1a 103.2 0.58 230 Reduced Catalyst 1b 168.3 0.57 143
Oxidic Composition 1b 122 0.57 196 Reduced Catalyst
EXAMPLE 2
Preparation of Catalysts
[0045] Example 1 was repeated except that no alumina trihydrate
slurry was fed to the co-precipitation reactor. The
co-precipitation was carried out by simultaneously adding 1.5 molar
sodium carbonate solution (3 litres) and an aqueous solution
containing 709.5 g Co nitrate hexahydrate, 294.2 g Zn nitrate
nonahydrate, 227.6 g Al nitrate nonahydrate and 38.2 g Mg nitrate
hexahydrate in 4180 ml demineralised water to the co-precipitation
reactor. Each stream was controlled so that the pH of the combined
solution was 6.9 and the temperature was 65.degree. C. After
precipitation the composition was again recovered by filtration and
washed with hot demineralised water to remove traces of sodium,
then dried at 105.degree. C. for 16 hrs and calcined at 300.degree.
C. for 6 hours. The composition of the un-reduced catalyst was as
follows;
TABLE-US-00004 % Wt Example 2 Co 42.2 Zn 15.2 Al 4.1 Mg 0.2 % Al on
ZnO 21.9
[0046] The catalyst was reduced at 425.degree. C. and subjected to
cobalt surface area analysis as described above. The oxidic
composition and the reduced catalyst were also analysed for BET
surface area and pore size/volume using nitrogen absorption. The
results are as follows;
TABLE-US-00005 WLOR Co Surface Area Co Surface Area Example (% w/w)
(m.sup.2/g cat) (m.sup.2/g Co) 2 21 26.4 49.5
[0047] Again the Co surface area/g cobalt is >20 m.sup.2/g
cobalt
TABLE-US-00006 BET Surface area Pore Volume Average Pore Example
(m.sup.2/g) (ml/g) Diameter (.ANG.) 2 165 0.42 103 Oxidic
Composition 2 129 0.65 204 Reduced Catalyst
EXAMPLE 3
Catalyst Testing
[0048] The catalysts were pre-reduced using hydrogen at or above
3000 hr.sup.-1 GHSV at a temperature of 425.degree. C. for 4 hours.
Catalysts were tested for the Fischer-Tropsch synthesis of
hydrocarbons in a 1-litre CSTR at a hydrogen to carbon monoxide
ratio of 2:1 at a temperature of 210.degree. C. and pressure of 20
bar abs. 3-5 g of pre-reduced catalyst was dispersed in a
hydrocarbon wax of average molecular weight 3000 (Polywax 3000) and
the gases introduced under pressure. The space velocity of the
gases was adjusted to give ca 50% CO conversion and measurements
taken at 190-200 hrs of activity, based on the normalised space
velocity required for 50% CO conversion per gram catalyst, and
selectivity for C5+ hydrocarbons. The results are follows;
TABLE-US-00007 Space velocity for 50% CO conversion per gram
catalyst C5+ selectivity Example (nl/hr g) % 1b 4.63 79.28 2 1.53
75.58
[0049] Thus the catalysts of the present invention are both active
and selective. Example 2 containing magnesium had a lower activity
that the Mg-free catalyst.
COMPARATIVE EXAMPLE C1
Omission of Aluminium
[0050] Cobalt-zinc compositions containing 25 and 45% wt Co (as
oxides) were prepared according to the method of example 2 but
without any aluminium compounds. The co-precipitates were aged for
6 hours at temperatures of 30 or 60.degree. C. before filtration
and washing. The samples were calcined at 250.degree. C. for 4
hours and the calcined oxidic materials reduced at 425.degree. C.
and their cobalt surface areas determined using the method
described above. The results are as follows;
TABLE-US-00008 Ageing Temp. Time % wt Co % wt Co WLOR Co Surface
Area Co Surface Area (.degree. C.) (h) Oxidic Reduced (%) m.sup.2/g
catalyst m.sup.2/g cobalt 30 6 25.0 35.7 30 4.5 12.6 60 6 25.0 36.2
31 3.6 9.9 30 6 45.0 72.6 38 1.0 1.4 60 6 45.0 73.8 39 1.5 2.0
[0051] These results show that in the absence of aluminium, very
low cobalt surface areas per gram catalyst or per gram cobalt are
obtained.
COMPARATIVE EXAMPLE C2
[0052] ZnO/Al supports were prepared according to the methods
described in Examples (a) and (b) of EP-0671976-B1. Cobalt nitrate
was impregnated onto these supports according to the method of
Example (e) of the same patent. The compositions of the un-reduced
catalysts obtained were as follows;
TABLE-US-00009 Wt % Example C2a Example C2b Co 7.61 7.84 Zn 65.36
64.62 Al 0.39 0.83 % Al on ZnO 0.48 1.03
[0053] The catalysts were reduced at 425.degree. C. and the cobalt
surface areas determined using the above method. The cobalt surface
areas were as follows;
TABLE-US-00010 Example C2a Example C2b Co Surface area (m.sup.2/g
Catalyst) 1.8 3.0
[0054] These catalysts have a very low cobalt surface area/g
catalyst.
[0055] Catalysts with such low surface areas would be expected to
show low activity for hydrogenation and Fischer Tropsch
reactions.
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